What is this?

This is hanging on the side workbench. It is under the top of the table hanging about 12 inches in. It slide to the left or right on a 3 foot hanger and can slide off the “hanger”. I think the “hanger”.has more to do with what it does than just for storage.

the pix shows that the block can be moved up or down. I have found nothing else to help it make work. The top left hand corner shows a bench dog stick through the bench.